Fibers prepared by using a thermoplastic polymer such as polyester or polyamide have excellent dynamic properties and size stability. Accordingly, these fibers are widely used not only in clothing applications, but also in automobile interior applications as well as industrial applications, and their industrial values are very high. However, the properties required for these fibers have diversified with the diversification of the textile applications, and existing polymers are often incapable of responding to these requirements. In such situations, designing a new fiber from scratch, namely, from the molecular level is associated with cost and time problems, and development of a composite fiber having the properties of two or more polymers is often selected. In such composite fibers, properties including sensory effects such as texture and bulkiness and mechanical properties such as tensile strength, initial modulus, and abrasion resistance that can not be realized by the single use of the main ingredient can be realized, for example, by coating the main ingredient with another ingredient. Various composite fibers with varying morphologies have been suggested, and various technologies have been proposed depending on the intended application of the fiber. Of these composite fibers, technical development is active in the field of so called “island-in-a-sea composite fibers” which are fibers having many island components arranged in the sea component.
Typical use of the island-in-a-sea composite fiber is the use as ultrafine fibers. In this case, the island-in-a-sea composite fiber is generally produced by arranging the island components comprising a hardly soluble component in the sea component comprising an easily soluble component, and removing the easily soluble component from the fiber or from the textile product prepared from the fiber to thereby produce an ultrafine fiber comprising the island component. In these days, ultimately thin ultrafine fiber of nano order level that can not be realized by the spinning of a single fiber can be prepared by using this technique, and the ultrafine fiber as thin as several hundred nm exhibits soft texture and flexibility that can never be realized by ordinary fibers. By using such properties, these ultrafine fibers have been developed, for example, as artificial leathers and textiles having new textures. Other applications include high density fabrics prepared by utilizing fiber interval compactness, and these high density fabrics are used, for example, in sport gear requiring wind protection and water repellency. The ultrafine fibers are capable of entering into minute grooves, and increasing the specific surface area, and dirt is caught in the fine gaps between the fibers. Accordingly, this fabric has high absorption and dust collecting ability. In the applications of industrial material, this property is used for wiping cloth and precision polishing cloth of precision machines.
The island-in-a-sea composite fibers which may be used for the production of the ultrafine fibers are generally divided into two types of fibers. One is polymer alloy type fibers produced by melt-kneading the polymers, and the other is those produced by composite spinning by using a composite nozzle. Of these composite fibers, those produced by the composite spinning are excellent since accurate control of the cross section of the composite fibers is enabled.
Various techniques are disclosed for the composite spinning of the island-in-a-sea composite fibers. Exemplary such techniques include those using a composite nozzle such as Japanese Patent Application Laid-Open No. H8-158144 (Claims) and Japanese Patent Application Laid-Open No. 2007-39858 (pages 1 and 2).
In Japanese Patent Application Laid-Open No. H8-158144 (Claims), a reservoir of the polymer (easily soluble component) with dilated cross section is provided under the hole of the hardly soluble component, and a core-sheath composite flow is thereby formed by inserting the hardly soluble component in the easily soluble component. After combining a plurality of such core-sheath composite flows, the combined flow is drawn and ejected to the final hole. In this technique, pressure of both the hardly soluble component and the easily soluble component are controlled by the size of the flow path between the flow dividing flow path and the introductory hole to thereby realize consistent pressure at the entrance of the introductory hole. The amount of the polymer ejected from the introductory hole is thereby regulated. Such a manner of controlling the pressure of the introductory holes to the same pressure is a good method in view of controlling the polymer flow. However, if the size of the island component is to be finally reduced to the level of nano order, the polymer flow rate should be reduced to the level as low as 10−2 g/min/hole to 10−3 g/min/hole at least for the sea component side introductory hole. In this case, the pressure loss which is in proportional relationship to the polymer flow rate and the wall interval becomes substantially zero, and accurate control of the sea component and island component polymers is very difficult. As a matter of fact, the ultrafine fibers generated from the island-in-a-sea composite fiber produced in the Examples is approximately 0.07 to 0.08 d (about 2700 nm), and the ultrafine fiber of nano order level is not yet obtained.
Japanese Patent Application Laid-Open No. 2007-39858 (pages 1 and 2) discloses that an island-in-a-sea composite fiber having fine hardly soluble components arranged in the cross section of the composite fiber is produced by repeating drawing and combining the composite flow wherein the easily soluble components and the hardly soluble components are arranged at a relatively equal interval. In this approach, the island component may be regularly arranged in the inner layer portion of the cross section of the island-in-a-sea composite fiber. However, shear force is applied to the outer layer portion by the nozzle wall during the drawing of the composite flow, and the flow rate is disturbed on the cross section being drawn. Large difference in the fiber diameter and morphology of the hardly soluble component is generated between the outer layer and the inner layer of the composite flow. In Japanese Patent Application Laid-Open No. 2007-39858 (pages 1 and 2), the procedure as described above has to be repeated over and over before the final ejection if the nano order level island component is to be produced. Accordingly, a large difference in the cross-sectional direction may be formed in the distribution of the morphology of the composite fiber, and this difference results in the variety of the diameter and the cross-sectional morphology of the island.
In the case of Japanese Patent Application Laid-Open No. 2007-100243 (pages 1 and 2), the nozzle technology used is the conventional known pipe-type island-in-a-sea composite nozzle. However, ratio of the melt viscosity between the easily soluble component and the hardly soluble component is defined to enable production of an island-in-a-sea composite fiber having a relatively controlled cross-sectional morphology. Japanese Patent Application Laid-Open No. 2007-100243 (pages 1 and 2), also describes that an ultrafine fiber having a consistent fiber diameter is produced by dissolving the easily soluble component in the post-processing step. In this approach, however, the hardly soluble component is finely divided by a group of pipes into minute flows, and these flows are supplied to core-sheath composite forming holes to produce core-sheath composite flows, and the composite flows are combined and drawn to form the island-in-a-sea composite fiber. The thus formed core-sheath composite flows of the number substantially corresponding to the number of islands are formed into a bundle, this bundle is drawn in an ejection plate having tapered holes formed therethrough to compress in the cross-sectional direction of the fiber for ejection from the ejection hole. In this stage, the fiber cross section is greatly compressed to 1/500 to 1/3000 and, accordingly, the core-sheath composite flows are compressed by interfering with each other. As a consequence, the cross section of the flow ejected from the composite forming hole attempts to become a perfect circle by the surface tension, while interference with other composite flows result in the deformed cross-sectional morphology of the island component and, therefore, intentional control of the island component is very difficult. Accordingly, consistency of the cross-sectional morphology was realized only to a limited extent. Such a limit is due to the principle of the conventional pipe-type nozzle that a bundle is formed by collecting the core-sheath composite flow that had been formed, and drawing the bundle, and only minimal effect can be expected to adjust the pipe configuration and arrangement. Accordingly, formation of a fiber having perfect circle cross section with consistent cross-sectional morphology was extremely difficult by using known approaches such as in Japanese Patent Application Laid-Open No. 2007-100243 (pages 1 and 2).
The island-in-a-sea composite fiber wherein 2 or more types of polymers are present in the cross section is inherently associated with the problem of unstable behavior in the deformation upon elongation of the fiber, and this instability is likely to be amplified when the island component has inconsistent cross-sectional morphology. The island-in-a-sea composite fiber did not have the stability of a common single fiber, and the conditions which can be used in the post-processing had been limited. When the sea is removed to generate the ultrafine fibers, the inconsistency and variety of the island component often invited partial deterioration of the island component both between the island components and along the fiber axis of the island components, and this often invited loss of the island component in the course of the post-processing step. This situation is not negligible in the island-in-a-sea composite fiber where the island component has achieved nano order level ultimate thinness since it greatly affects whether the fiber and the textile products produced therefrom can endure the post-processing step as well as their properties. In view of such a situation, there is a strong demand for the development of an island-in-a-sea composite fiber having an extremely thin island component with nano order diameter wherein the island component is a perfect circle and the cross-sectional morphology is consistent.
It could therefore be helpful to provide an island-in-a-sea composite fiber wherein the island component is an extremely thin fiber having a nano-order diameter, and the fiber has consistent morphology with perfect circle cross section.